Toward sparse and geometry adapted video approximations

Video signals are sequences of natural images, where images are often modeled as piecewise-smooth signals. Hence, video can be seen as a 3D piecewise-smooth signal made of piecewise-smooth regions that move through time. Based on the piecewise-smooth model and on related theoretical work on rate-distortion performance of wavelet and oracle based coding schemes, one can better analyze the appropriate coding strategies that adaptive video codecs need to implement in order to be efficient. Efficient video representations for coding purposes require the use of adaptive signal decompositions able to capture appropriately the structure and redundancy appearing in video signals. Adaptivity needs to be such that it allows for proper modeling of signals in order to represent these with the lowest possible coding cost. Video is a very structured signal with high geometric content. This includes temporal geometry (normally represented by motion information) as well as spatial geometry. Clearly, most of past and present strategies used to represent video signals do not exploit properly its spatial geometry. Similarly to the case of images, a very interesting approach seems to be the decomposition of video using large over-complete libraries of basis functions able to represent salient geometric features of the signal. In the framework of video, these features should model 2D geometric video components as well as their temporal evolution, forming spatio-temporal 3D geometric primitives. Through this PhD dissertation, different aspects on the use of adaptivity in video representation are studied looking toward exploiting both aspects of video: its piecewise nature and the geometry. The first part of this work studies the use of localized temporal adaptivity in subband video coding. This is done considering two transformation schemes used for video coding: 3D wavelet representations and motion compensated temporal filtering. A theoretical R-D analysis as well as empirical results demonstrate how temporal adaptivity improves coding performance of moving edges in 3D transform (without motion compensation) based video coding. Adaptivity allows, at the same time, to equally exploit redundancy in non-moving video areas. The analogy between motion compensated video and 1D piecewise-smooth signals is studied as well. This motivates the introduction of local length adaptivity within frame-adaptive motion compensated lifted wavelet decompositions. This allows an optimal rate-distortion performance when video motion trajectories are shorter than the transformation "Group Of Pictures", or when efficient motion compensation can not be ensured. After studying temporal adaptivity, the second part of this thesis is dedicated to understand the fundamentals of how can temporal and spatial geometry be jointly exploited. This work builds on some previous results that considered the representation of spatial geometry in video (but not temporal, i.e, without motion). In order to obtain flexible and efficient (sparse) signal representations, using redundant dictionaries, the use of highly non-linear decomposition algorithms, like Matching Pursuit, is required. General signal representation using these techniques is still quite unexplored. For this reason, previous to the study of video representation, some aspects of non-linear decomposition algorithms and the efficient decomposition of images using Matching Pursuits and a geometric dictionary are investigated. A part of this investigation concerns the study on the influence of using a priori models within approximation non-linear algorithms. Dictionaries with a high internal coherence have some problems to obtain optimally sparse signal representations when used with Matching Pursuits. It is proved, theoretically and empirically, that inserting in this algorithm a priori models allows to improve the capacity to obtain sparse signal approximations, mainly when coherent dictionaries are used. Another point discussed in this preliminary study, on the use of Matching Pursuits, concerns the approach used in this work for the decompositions of video frames and images. The technique proposed in this thesis improves a previous work, where authors had to recur to sub-optimal Matching Pursuit strategies (using Genetic Algorithms), given the size of the functions library. In this work the use of full search strategies is made possible, at the same time that approximation efficiency is significantly improved and computational complexity is reduced. Finally, a priori based Matching Pursuit geometric decompositions are investigated for geometric video representations. Regularity constraints are taken into account to recover the temporal evolution of spatial geometric signal components. The results obtained for coding and multi-modal (audio-visual) signal analysis, clarify many unknowns and show to be promising, encouraging to prosecute research on the subject

Bottom-up approaches for organizing nanoparticles with polymers

This thesis describes some of three years'work carried at the Centre Suisse d'Electronique et de Microtechnique (Neuchâtel, Switzerland) under the supervision of Dr. Martha Liley and in collaboration with Prof. Horst Vogel of the Ecole Polytechnique Fédérale of Lausanne (Switzerland). The goal of this work is to contribute to the development of innovative technologies in the treatment of surfaces, for the lateral organization of various materials on the micrometer and nanometer scale. The aim is therefore to offer a valid alternative to common lithographic techniques that often require expensive capital equipment and infrastructures. In spite of the high-quality and precision of their products, these techniques often have a limit of ~70 nm on the resolution of the minimum feature size. In this work, techniques based on the ability of some systems to self-organize were used. In particular, the tendency of immiscible polymer mixtures and of block copolymers to form separate phases was exploited. Films made of different polymer phases formed templates for the organization of various materials, with length scale that varied from some 10 µm to some 10 nm. The materials that were most investigated were particles of the diameter of some nanometer, made of semiconductors (CdSe and ZnS) and of metals (Au, CoPt3 and Co). These nanoparticles have different properties from the corresponding bulk material. This fact is due to two main phenomena: in a nanoparticle the electronic energy levels of electrons are discrete because of their spatial confinement. Also, surface effects arise due to the high surface to volume atom ratio compared to the bulk material. As an example, semiconductor particles of the diameter of some nanometers are fluorescent. Semiconductor nanoparticle fluorescence is usually characterized by long fluorescence lifetimes. This fact promoted the investigation, in the framework of this thesis, of the fluorescence properties of Mn-doped ZnS nanoparticles. These particles turned out to have exceptionally long fluorescence lifetimes compared to the fluorophores commonly used as fluorescent markers in biological techniques. This singular property allowed the construction of a simple and cost effective instrument for time-resolved detection of the nanoparticle fluorescence. This technique allows a remarkable increase of the signal-to-noise ratio compared to conventional detection methods. Two approaches were explored to laterally organize nanoparticles on polymer surfaces. In the first method, the nanoparticles were added into a mixture of immiscible polymers and a film was formed from the solution by spin-coating, typically on silicon or glass slides. Numerical simulations by other authors indicate that if the particles have a higher affinity for one of the two polymers, they will be distributed in the corresponding phase. This behaviour was confirmed by our experiments. The lateral dimensions of the patterns in which the nanoparticles organized could be changed with continuity from some 10 µm to se sub-micrometer range. Their shape was modelled from a stochastic to an ordered type. The second approach explored for the organization of the nanoparticles consisted of the pre-formation of the polymer films and their subsequent decoration with the nanoparticles. Due of the different interactions of the two polymers with the particle-solvent system, different absorption behaviours of the nanoparticles were found on the two polymer phases. This technique allowed nanoparticle organization on homopolymer demixed films in patterns having typical sizes in the micron and in the submicron range. Alternatively, the nanoparticles were organized on block copolymer films in regular patterns having typical periodicities of the order of 100 nm. Nanoparticles organized on thin block copolymer films could be transferred on the hard substrate via removal of the polymer molecules by oxygen plasma etching. This process did not affect the nanoparticle organization. Under particular conditions, an aggregation of 10 nm gold nanoparticles was induced using oxygen plasma. This technique allowed the formation of gold nanowires and nanostructures both on polymer layers and on the hard substrate. Their width varied from about 25 nm to the micrometer, while their length extended for various micrometers. They presented a fingerprint like structure or, alternatively, quasi-parallel nanowires extended for several µm2, the typical periodicities being about 100 nm. The conductivity of these nanowires and nanostructures was demonstrated using SEM

CHRONIQUE

à la Faculté des études supérieures et postdoctorales de l'Université Laval dans le cadre du programme de doctorat en médecine expérimentale pour l'obtention du grade de Philosophiae Doctor (Ph.D.

DÉPARTEMENT D'ANATOMIE ET DE PHYSIOLOGIE

à la Faculté des études supérieures de l'Université Laval dans le cadre du programme de doctorat en physiologie-endocrinologie pour l'obtention du grade de Philosophias Doctor (Ph.D.